In a case that a potential higher than a basic potential of unit battery (cell) is necessary, such as a hybrid vehicle using a lithium ion cell as a power source, it is common to use a plurality of unit batteries which is connected in series. However, even though the batteries are produced with the same structure via a typical production method using the same anode, cathode and electrolyte material, a difference in charging or discharging (and self discharging) characteristics exists between each of the batteries connected in series.
Therefore, a potential difference can exist between the unit batteries when using the batteries connected in series. Even if one battery of the unit batteries connected in series is perfectly discharged regardless of a potential of other battery, a total voltage (total voltage of batteries connected in series) becomes zero so that the battery is need to be recharged. Upon recharging the battery, since the potentials of the batteries are different from one another, there is a problem of over-charging in that the battery may reach a prescribed voltage in advance, as well as a problem of charge inefficiency in that some batteries may not reach the prescribed voltage even in an existence of an over-charging.
Further, if the number of charging/discharging times becomes large, degradation is caused in materials composing the battery and thus a property of the battery is varied, and as a result, such degradation situation is responsible for further increasing a difference in individual cells.
In order to address such problems, there are proposed various charge equalization apparatuses which can accomplish charge equalization of the batteries connected in series.
As an example, Korean patent Laid-Open No. 2003-0096978 is directed to a system consisted of a plurality of unit cells, a charging means, a discharging means, series-parallel converting switch, wherein each of the plurality of unit cells is uniformly discharged and then charged by connecting the discharged unit cells in series. Korean Patent Laid-Open 2007-0064244 is directed to a system which comprises a cell unit, a field effect transistor unit connected to the cell unit, an amplifying unit connected to the field effect transistor unit, a multiplexer unit controlling an output signal of the amplifying unit, a comparing unit comparing a difference between voltage signals of the cell unit, an A/D converter converting the output of the comparer to a digital signal, a Micom unit inputting the signal outputted by the A/D converter and outputting the signal corresponding to charging/discharging condition, a switching unit operated according to a signal of the Micom unit to provide battery equalization current, and a known charging/discharging unit.
Further, Japanese Patent Laid-Open No. 1998-032936 is related to a system consisted of a plurality of unit cells, a means for detecting remaining capacity of each unit cell, charge and discharge replacing means for performing charging and discharging of each unit cell, a controller controlling charging and discharging of each unit cell, and direct current/direct current converter performing charging and discharging of each unit cell independently. Japanese Patent Laid-Open No. 2004-194410 is related to a system consisted of at least two unit cell groups, a means for detecting a difference between currents flowing each of a first cell group, and a second cell group, a means for controlling charging/discharging current of the cell group based on the current difference. U.S. Patent Laid-Open No. 2007-0145946 is related to a system comprising a plurality of unit cells, a charge equalization circuit having a DC-DC converter, and a control circuit for controlling a voltage of each cell by monitoring the voltage.
The prior technology can be represented as FIG. 1, which shows a battery management system having a voltage sensor separated from a cell equalizer. N batteries are connected in series and a switch block is located in a right side of the batteries. The switch block is responsible for selecting the specific battery (cell) to provide a next-stage capacitor and a current path. The potential stored in the capacitor is read in via an A/D converter, which is then input to a microprocessor. The battery voltage information inputted is used to operate the battery at the microprocessor. The microprocessor accomplishes charge equalization of total batteries by driving the charge equalizer connected in parallel to each of batteries, if the specific battery is low-charged or over-charged.
As shown in FIG. 1, prior battery management system having the voltage sensor separated from the cell equalizer has a disadvantage of larger volume, since each battery (cell) of the series-connected battery string has an individual charge equalizer (cell equalizer) of the same function, and also a switch module for reading-in voltage of each battery and a switch module for controlling the charge equalizer are separately provided as the voltage sensor. Further, it has a disadvantage that a voltage stress of the switch is increased as total series-connected battery string is controlled by the switch module.